A. Technical Field
This application relates to optical signal power control, and more particularly, to feedback control of an optical gain or loss that is applied to an optical channel group(s).
B. Background of the Invention
The importance of optical networking technology in today's society is well understood. Optical networks transmit large amounts of information at high data rates. Optical networks may also transmit information across long distances using fiber optic cables. Typically, multiple optical wavelengths or channels are multiplexed together and transmitted into this cable. The power level of these optical channels should be maintained within a preferred range to ensure that information within these channels may be efficiently recovered.
Optical amplifiers or attenuators provide a gain to a signal that effectively increases or decreases the power associated with the signal. These optical amplifiers may be used to ensure that a power level or optical profile of an optical signal falls within a preferred power range. Certain optical components, such as photo-detectors and optical multiplexers, are designed to operate within a particular range of optical power. If an optical signal is not within this power range, then the component may generate errors in its operation.
Optical multiplexers may require that an optical channel have certain power characteristics in order to properly multiplex that channel within a wave-division multiplexed signal. FIG. 1 illustrates an example of an optical multiplexing module that multiplexes multiple banded optical channels. As shown, a plurality of banded optical channel source modules 110 transmit banded optical channels onto paths 120 to a multiplexing module 150. An N:1 multiplexer 160 receives each of the banded optical channels 120 and combines them into a single wave-division multiplexed signal 170.
The output characteristics of each of the banded optical channel group source modules 110 may vary depending on the vendors, ages and operating temperatures of modules 110. Furthermore, the optical paths 120 typically have different transmission characteristics which may affect the optical signals differently. As a result, the banded optical signals may have different characteristics, such as average power levels, at the multiplexer 160.
FIG. 2 illustrates an exemplary plot of the optical profile of the banded optical channel groups at the multiplexer 160. The plot shows a preferred power level 210 for the multiplexer 160. A first banded optical channel group, having a first set of channels 230, is shown as having the preferred power level 210. A second banded optical channel group, having a second set of channels 240, is shown as having a power level above the preferred power level 210. A third banded optical channel group, having a third set of channels 250, is shown as having a power level below the preferred power level 210.
When combining optical channels together, it may be desirable to have the channels have equal power for flatness or to purposefully render them to have a relative power profile that is of a certain desired shape. An undesired variable optical power profile at the multiplexer 160 may cause the multiplexer 160 to operate outside of a preferred power level range and may potentially result in the generation of errors. Desired channel “pre-emphasis” is typically done by an optical amplifier or attenuator on a channel-by-channel basis. For example, channel pre-emphasis may be performed by making use of a spectrum analyzer or per channel power meter to provide the feedback needed to create the change in powers be it by an amplifier or attenuator.
This optical power profile may change overtime as the banded optical channel group source modules age and/or the characteristics of the transmission paths 120 vary. For example, a laser's output characteristics may change of time as it ages or its temperature increases. This change in laser output may cause an optical channel group power level to wander outside of a preferred power range.